1. Field of the Invention
The invention relates in general to a charging device, and more particularly to a two-stage charging device.
2. Description of the Related Art
In order to make the electronic products more compact and light, developers are searching for a breakthrough in the design of portable electronic devices. The design of a notebook computer, for example, not only needs to be compact, but also requires considerations about the battery duration and heat dissipation problems. How to miniaturize the product and at the same time reduce its heat dissipation thus becomes an important issue.
Portable electronic devices normally use batteries as the power supply; and the rechargeable battery module is normally adopted. The design of a charging device is particularly important when both the efficiency and the stability of power supply are considered. The charging device typically has a power supply device and a control device, wherein the power supply device serves as a necessary power supply during charging of the battery, and the control device controls the charging process. The most commonly used power supply devices fall into two categories: the linear voltage regulator and the switching voltage regulator, with which respective advantages and disadvantages are associated. Despite having the advantage of a simple circuit design, the linear voltage regulator has a low efficiency when the voltage difference between the input voltage and the output voltage is large. In contrast to the linear voltage regulator, the switching voltage regulator, despite having a higher efficiency, has a complicated circuit design which is unfavorable for the miniaturization of the circuit.
FIG. 1 is a schematic diagram of a conventional linear charging device. As shown in FIG. 1, the power supply device 110 has a main power supply Vs for charging a battery BT The battery BT has a battery voltage Vb. When battery voltage Vb drops too low, the drive device 130 enables the transistor Q, allowing the battery BT to be charged by the main supply voltage Vs via the transistor Q. Along with the passing of the charging time, the battery voltage Vb of the battery BT rises. When the battery BT is saturated, the drive device 130 cuts off the power supply Vs to the battery BT by disabling the transistor Q. This method may appear to be simple. However, charging efficiency may be significantly reduced when the transistor Q consumes a large amount of energy because of large difference between the main supply voltage Vs and the battery voltage Vb. In order to use precious energy more efficiently and to prevent the transistor Q from being destroyed when the transistor Q is overloaded, the conventional charging device needs to be further improved.